Breeding of Corn Plants
Corn plants (Zea mays L.) are bred by both self-pollination and cross-pollination techniques. Corn is a monoecious plant, i.e., each plant has separate male and female flowers on the same plant, located on the tassel and ear, respectively. Natural pollination occurs in corn when pollen is shed from tassels and contacts silks of the same plant or a different plant that protrude from tops of the developing ears. Methods and techniques for the development of inbred corn lines and hybrid corn varieties are known in the art. Hallauer, A., Maize, in Principles of Cultivar Development, Vol. 2, Fehr. W. ed. pp. 249-294, Macmillan, New York, (1987). Currently, many hybrid corn varieties are produced by crossing two inbred lines to produce F.sub.1 hybrid progeny. The F.sub.1 plants exhibit heterosis, or hybrid vigor, resulting in plants having high yield and superior agronomic performance in the hybrid combination. The production and development of inbred corn lines and hybrid corn varieties are discussed in, for example, U.S. Pat. No. 5,367,109, which is incorporated herein by reference.
Research studies on maize have resulted in the identification of numerous genetic loci. See, e.g., the maize genetic database on the Worldwide Web at http://teosinte.agron.missour.edu/top.html.
To supply the need nutrients for increased milk production by lactating dairy cattle is an ongoing challenge facing the dairy industry. This challenge is complicated by the fact that, even though a dairy cow's diet may meet the National Research Council recommended nutrient requirements, the diet may still lack some nutrients at increased levels required for higher milk production. One reason for this difficulty in meeting nutrient requirements is the complexity of the digestive system of ruminants such as dairy cattle.
In cattle, ingested feed first passes into the reticulorumen, where it is subject to anaerobic microbial fermentation. Microbial fermentation begins the digestive process and gives a ruminant the ability to utilize fibrous feeds, in contrast to monogastric animals. Ruminants meet their nutrient needs by utilizing the by-products of microbial fermentation, along with any undigested feed residues and the resultant microbial mass that passes from the rumen.
Anaerobic microbial fermentation is an advantage to ruminants because it allows them to benefit from feeds which cannot be utilized by non-ruminants. However, microbial activity limits the ability to provide supplemental nutrients to a ruminant animal, because many desirable nutrients, such as proteins, amino acids and digestible fiber, will be metabolized by microbes before the nutrients reach a site where they can be absorbed and utilized by the ruminant.
Attempts have been made to increase milk production in dairy cattle by manipulating the feed ration. For example, rations containing silage derived from corn plants carrying a brown midrib mutation have been fed to cattle. Stallings, C. et al., J. Dairy Sci., 65:1945-1949 (1982); Block, E. et al., J. Dairy Sci. 64:1813-1825 (1981); Keith, E. et al., J. Dairy Sci. 62:788 (1979). The bm gene decreases and alters the lignin content in the vegetative parts of such corn plants; silage made from such plants has increased fiber digestibility compared to silage from corn plants not exhibiting the brown midrib phenotype. In general, these studies indicated that there was no increase in milk production in cows fed silage from bm corn. It was concluded that the cows fed a diet containing bm silage partitioned the nutrients into meat or fat body tissues rather than milk production. Barriere et al., Agronomie 13:865-876 (1993).
Attempts have been made to increase the efficiency of feed utilization and milk production by using various formulations and feed supplements. Despite continued improvement in the development of dairy cattle feed rations, it is desirable to further increase the efficiency of feed utilization and milk production by dairy cattle.